Beginning in 1934, with the issuance of U.S. Pat. No. 1,979,402, entitled "Concentric Shield for Cables" (Nyquist Patent), the communications industry has shown renewed interest in increasing channel capacity in what is now referred to as Pulse Code Modulation carrier systems (PCM systems). The Nyquist Patent directed itself to the problem of transmitting signals of the same frequency in two directions within the same cable in the so-called carrier frequency range. It disclosed: that it was desirable that the conductors used for transmitting signals in one direction be electrically shielded from those transmitting signals in the opposite direction; that shielding material could be a thin tape of soft iron, alternating with layers of copper; a theoretical rationale and mathematic equations to support the conclusion that the product of the permeability and the conductivity of the iron is large, therefore, making its attenuating effect also large, thus having a shielding effect of a desirable magnitude; and that the ratio of the permeability of the iron to its conductivity is quite different from that of copper or other conductive materials and that the combination of iron and copper in alternating layers (no metallurgical bonding disclosed) caused electromatic wave reflection loss brought about by interfering waves. For the sake of completeness, the disclosure of Nyquist's teaching (U.S. Pat. No. 1,979,402) is incorporated by reference.
In 1937, an Australian Pat. No. (105,876) issued which essentially duplicated the teachings of Nyquist and went on further to disclose a telephone cable having a core that was divided into two groups, each group being essentially completely surrounded by alternating layers of iron and aluminum. The two groups of conductors were "D shaped" in cross section and fitted together so that the composite core had a cross section that was essentially circular.
A great deal of time thereafter transpired before any significant additional attention was given to the problem of near-end cross-talk, such as that addressed by Nyquist and the Australian Patent.
In 1971, there appeared a patent to Roberts et al. (U.S. Pat. No. 3,622,683), which might be identified as the beginning of the modern day interest relative to internally screened cable for for use in PCM communications systems. This patent and those that followed, an example of which is U.S. Pat. No. 4,085,284, all addressed themselves to the problem of near-end cross-talk associated with internally screened cables in PCM systems. These disclosures, beginning with Nyquist up to the present time, never addressed the problem of armoring the screen of an internally screened cable not only to provide the screening function between conductor but also to protect the conductors from mechanical forces that would otherwise destroy the integrity of the cable and cracking of the internal screen as a result of bending or flexing of the cable along anyone of its axes. The Roberts et al. teaching does disclose a conventional armoring means to protecting a cable from outside forces (armoring) in the form of a metal tape folded around the outer periphery of a cable core. This metal tape was also denoted as a metal shield and such shielding was in addition to still another shield, an internal radially extended shield that longitudinally divided the cable core into two groups.
Corrugation of a single metal internal screen in an internally screened cable is disclosed by French Pat. No. 852,028.
The instant invention addresses itself to the problems associated with internally screened cable, problems of cracking of internal screens when flexed either during installation or service and the integrating of a protective armor layer within the screen itself. Both the shielding and armor protection characteristics of the invention arise out of a single metallic tape shield composed of two layers of aluminum or copper bonded to both surfaces of a middle iron or steel layer. The tape has a medial portion integrally joined to two terminal portions. The cable core is made up of two groups of conductors divided one from the other by the medial portion of the shielding tape. The terminal portions are bent in opposite directions and lie on the periphery of opposing groups and extend to and beyond that point where the medial and terminal portions merge, thus disposing the terminal free edges of the single bimetallic screen on circumferentially spaced apart portions of the cable core. One feature of the invention calls for the shield to be corrugated throughout. It is believed that the corrugations contribute to and/or are the basis for more than one of the desirable features of the invention described in the more detailed portion of this disclosure.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.